Nanomolar detection with high sensitivity microfluidic absorption cells manufactured in tinted PMMA for chemical analysis

We describe a novel, cost effective and simple technique for the manufacture of high sensitivity absorption cells for microfluidic analytical systems. The cells are made from tinted polymethyl methacrylate (PMMA) in which microfluidic channels are fabricated. Two windows (typically 250 ?m thick, resulting in little optical power loss) are formed at either end of the channel through which light is coupled. Unwanted stray light from the emitter passes through a greater thickness of the tinted substrate (typically the length of the cell) and is preferentially absorbed. In effect, this creates a pin-hole configuration over the length of the absorption cell, providing improved performances (sensitivity, S/N ratios, baseline noise and limit of detection) when used as an absorption cell compared to clear substrates. The method is used to achieve a LOD of 20 nM with a colourimetric iron assay and a LOD of 0.22 milli-absorption units with a pH assay

Characterisation and deployment of an immobilised pH sensor spot towards surface ocean pH measurements

International audienceThe oceans are a major sink for anthropogenic atmospheric carbon dioxide, and the uptake causes changes to the marine carbonate system and has wide ranging effects on flora and fauna. It is crucial to develop analytical systems that allow us to follow the increase in oceanic pCO2 and corresponding reduction in pH. Miniaturised sensor systems using immobilised fluorescence indicator spots are attractive for this purpose because of their simple design and low power requirements. The technology is increasingly used for oceanic dissolved oxygen measurements. We present a detailed method on the use of immobilised fluorescence indicator spots to determine pH in ocean waters across the pH range 7.6–8.2. We characterised temperature (−0.046 pH/°C from 5 to 25 °C) and salinity dependences (−0.01 pH/psu over 5–35), and performed a preliminary investigation into the influence of chlorophyll on the pH measurement. The apparent pKa of the sensor spots was 6.93 at 20 °C. A drift of 0.00014 R (ca. 0.0004 pH, at 25 °C, salinity 35) was observed over a 3 day period in a laboratory based drift experiment. We achieved a precision of 0.0074 pH units, and observed a drift of 0.06 pH units during a test deployment of 5 week duration in the Southern Ocean as an underway surface ocean sensor, which was corrected for using certified reference materials. The temperature and salinity dependences were accounted for with the algorithm, R=0.00034−0.17·pH+0.15·S2+0.0067·T−0.0084·S·1.075R=0.00034−0.17·pH+0.15·S2+0.0067·T−0.0084·S·1.075. This study provides a first step towards a pH optode system suitable for autonomous deployment. The use of a short duration low power illumination (LED current 0.2 mA, 5 μs illumination time) improved the lifetime and precision of the spot. Further improvements to the pH indicator spot operations include regular application of certified reference materials for drift correction and cross-calibration against a spectrophotometric pH system. Desirable future developments should involve novel fluorescence spots with improved response time and apparent pKa values closer to the pH of surface ocean waters

Development of a colorimetric microfluidic pH sensor for autonomous seawater measurements

High quality carbonate chemistry measurements are required in order to fully understand the dynamics of the oceanic carbonate system. Seawater pH data with good spatial and temporal coverage are particularly critical to apprehend ocean acidification phenomena and their consequences. There is a growing need for autonomous in situ instruments that measure pH on remote platforms. Our aim is to develop an accurate and precise autonomous in situ pH sensor for long term deployment on remote platforms. The widely used spectrophotometric pH technique is capable of the required high-quality measurements. We report a key step towards the miniaturization of a colorimetric pH sensor with the successful implementation of a simple microfluidic design with low reagent consumption. The system is particularly adapted to shipboard deployment: high quality data was obtained over a period of more than a month during a shipboard deployment in northwest European shelf waters, and less than 30 mL of indicator was consumed. The system featured a short term precision of 0.001 pH (n = 20) and an accuracy within the range of a certified Tris buffer (0.004 pH). The quality of the pH system measurements have been checked using various approaches: measurements of certified Tris buffer, measurement of certified seawater for DIC and TA, comparison of measured pH against calculated pH from pCO2, DIC and TA during the cruise in northwest European shelf waters. All showed that our measurements were of high quality. The measurements were made close to in situ temperature (+0.2 ?C) in a sampling chamber which had a continuous flow of the ship’s underway seawater supply. The optical set up was robust and relatively small due to the use of an USB mini-spectrometer, a custom made polymeric flow cell and an LED light source. The use of a three wavelength LED with detection that integrated power across the whole of each LED output spectrum indicated that low wavelength resolution detectors can be used instead of the current USB mini spectrophotometer. Artefacts due to the polychromatic light source and inhomogeneity in the absorption cell are shown to have a negligible impact on the data quality. The next step in the miniaturization of the sensor will be the incorporation of a photodiode as detector to replace the spectrophotometer

Seawater-pH measurements for ocean-acidification observations

The uptake of anthropogenic CO2 by the oceans since the onset of the industrial revolution is considered a serious challenge to marine ecosystems due to ensuing carbonate-chemistry changes (ocean acidification). Furthermore, the CO2 uptake is reducing the ocean’s capacity to absorb future CO2 emissions. In order to follow the changes in the ocean’s carbonate system, high-quality analytical measurements with good spatial and temporal resolution are necessary. High-precision and accurate pH measurements are now possible, and allow us to determine the progression of ocean acidification. The spectrophotometric pH technique is now widely used and capable of the required high-quality measurements. Spectrophotometric pH systems are deployed on ships and in situ on remote platforms. Smaller and more rugged instruments are nevertheless required for more widespread in situ application to allow routine high-resolution measurements, even in the most remote regions.We critically review oceanic pH measurements, and focus on state-of-the-art spectrophotometric pH measurement techniques and instrumentation. We present a simple microfluidic design integrated in a shipboard instrument featuring high accuracy and precision as a key step towards a targeted pH microsensor system

An automated microfluidic colourimetric sensor applied in situ to determine nitrite concentration

A stand-alone sensor system with integrated sub-systems is demonstrated. The system is portable and capable of in situ reagent-based nutrient analysis. The system is based on a low cost optical detection method, together with an automated microfluidic delivery system that is able to detect nitrite with a limit of detection (LOD) of 15 nM. The sensor was operated in situ at Southampton Dockhead for 57 h (December 2010) and 375 measurements were taken